A power conversion device is a device which converts input power to desired output power by controlling ON/OFF of an element called a switching element, and is utilized in various purposes such as power sources of various electronic devices and driving power supply for a motor or the like.
The switching element of the power conversion device is constituted of a semiconductor transistor and a diode connected in parallel to each other. For example, a power MOSFET (Metal Oxide Semiconductor-Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor) and others are used as this semiconductor transistor.
Single crystal Si (silicon) is generally used for semiconductor layers constituting the power MOSFET and IGBT. However, some MOSFETs use a wide band-gap semiconductor such as SiC (silicon carbide) having a band gap wider than that of Si for the semiconductor layers in recent years.
Also, as the transistor using a wide band-gap semiconductor, a transistor called a HEMT (High Electron Mobility Transistor) using a III-V compound semiconductor such as GaN (gallium nitride) has been known.
For example, Japanese Patent Application Laid-Open Publication No. 2006-86398 (Patent Document 1) discloses a transistor having a HEMT structure in which a buffer layer, a channel layer made of a III-V nitride semiconductor and a Schottky layer made of a III-V nitride semiconductor are formed on a conductive layer, and a source electrode, a drain electrode and a gate electrode are formed above the Schottky layer.